Use Gtk::Box for HBoxOperations (!25)

Gtk::HBox and Gtk::VBox were deprecated in Gtkmm 3.2.  Replace with
plain Gtk::Box.

This commit makes the change for HBoxOperations.{h,cc}.

Closes !25 - Modern Gtk3 - part 1

include/HBoxOperations.h

@@ -28,7 +28,7 @@
 namespace GParted
 {
 
-class HBoxOperations : public Gtk::HBox
+class HBoxOperations : public Gtk::Box
 {
 public:
 	HBoxOperations() ;

src/HBoxOperations.cc

@@ -23,7 +23,8 @@
 namespace GParted
 {
 
-HBoxOperations::HBoxOperations() 
+HBoxOperations::HBoxOperations()
+ : Gtk::Box(Gtk::ORIENTATION_HORIZONTAL)
 {
 	//create listview for pending operations
 	liststore_operations = Gtk::ListStore::create( treeview_operations_columns );